The present invention relates to a rolling mill and a rolling method and, particularly, to a rolling mill and rolling method each of which is suitable for rolling a metal plate required for particularly excellent quality.
In the field of plate rolling, improvement of the quality is always required, and various types of rolling mills have been proposed to improve precision in size of plates. For example, JP B 50-24903 and JP B 52-1701 each proposes a 6-high mill which is provided with work rolls, intermediate rolls and backup rolls in rolling stands and in which the intermediate rolls are arranged to cross an imaginary line perpendicular to a rolling direction (which is simply referred to such that the roll or rolls cross), and a rolling method thereby. According to the prior art, the intermediate rolls are arranged so that they are able to be inclined with respect to the imaginary line in a horizontal plane and plate thickness distribution in a plate width direction can be changed by changing the inclination angle, so that a large improvement of control ability for thickness distribution is expected. Further, since load from portions of the backup rolls that the work rolls and a plate are not in contact with each other can be decreased to a small load, it is expected that the plate thickness distribution in the plate width direction can be easily corrected for a short time by jointly using this inclination adjusting method and a roll bending method.
Further, JP A 61-279305 proposes a technique of using special rolls which enables axial shift of barrel sleeves of rolling rolls and effecting simultaneously axial shift of the rolls and crossing of the rolls. As for a 6-high mill, it discloses a rolling mill in which intermediate rolls and backup rolls, or the intermediate rolls and work rolls are arranged to cross as a unit. According to this conventional technique, it is expected to improve greatly a control ability of thickness distribution by enabling the rolling rolls to incline in a horizontal plane and changing the inclination angle, as in the above-mentioned technique.
A further technique of reducing the thrust force in a 6-high mill of a type in which work rolls cross is disclosed in JP A 6-31304. In this conventional technique, shape control is effected by crossing the work rolls of the 6-high mill, and the thrust force is measured and the intermediate rolls are crossed in a counter direction so as to cancel the thrust force.
On the other hand, an example of rolling mills put into practice as a rolling mill of a type in which rolling rolls cross is a 4-high mill of pair cross type in which a roll pair of upper work roll and upper backup roll and a roll pair of lower work roll and lower backup roll, each roll pair crosses as one roll pair unit, which is reported in "MITUBISHI HEAVY INDUSTRY TECHNICAL REPORT" Vol.21, No.6 (1984)", pages 61-67. Further, another example of rolling mills being put into practice is a 4-high mill in which only work rolls are enabled to cross and a lubricant supplying apparatus is provided for supplying lubricant between rolls, which is disclosed in JP A 5-50110.
In the 4-high mill, since the work rolls are driven, the work rolls of relatively large diameter are used. However, small diameter rolls are suitable for rolling of a hard material or a thin material. For such materials, multi-stage mills and 6-high mills are used in many cases.
As conventional rolling mills of a type using crossing of rolling rolls, there are 6-high mills and 4-high mills, as mentioned above. However, any rolling mills which have succeeded in practice are 4-high mills, and there is no example of a 6-high mill which has succeeded in practice. The reason is that occurrence of thrust force is a problem in the case of crossing rolling rolls and it is essential to decrease the thrust force in order to practice them.
In the 4-high mills disclosed in the "MITUBISHI HEAVY INDUSTRY TECHNICAL REPORT Vol. 21, No.6(1984)" and JP A 5-50110, any types of them employ a specific device to decrease axial thrust force caused by crossing the work rolls.
That is, as the axial thrust force caused by crossing the work rolls, there are axial thrust force caused between the work rolls and intermediate rolls and axial thrust force caused between the work rolls and a plate. In a 4-high mill of pair crossing type, the axial thrust force between the work roll and the backup, of them, is prevented to occur by arranging the backup rolls and the work rolls to cross along the same axis. Further, in the type of 4-high mill that crosses only the work rolls, the thrust force occurring between the work rolls and backup rolls is decreased by supplying lubricant between rolls, and cancelled by the thrust force occurred between the plate and the work rolls, whereby the thrust force occurring in the work rolls is decreased in total.
On the other hand, the JP B 50-24903, JP B 52-1701 and JP A 61-279305 each disclose 6-high mills, however, none of them discuss (discussion of reduction of the axial thrust force) concerning the axial thrust force caused by crossing rolling rolls, and does not take any special device for decreasing the axial thrust force caused by crossing rolls. Therefore, excessive large axial thrust force occurs on the work rolls and intermediate rolls during rolling, whereby the rolling becomes impossible to succeed any more.
In the 6-high mill disclosed in JP A 6-31304, such a device is employed that the thrust force occurring on the work rolls is measured and the intermediate rolls are crossed in a counter direction so as to cancel the thrust force. However, according to the study by the inventors of the present application, the thrust force can not be decreased substantially only by crossing the work rolls and intermediate rolls in a counter direction to each other, and it is found that the excessive large axial thrust force occurs still on the work rolls during rolling and the rolling become impossible to succeed further.
Further, the following fact was found. That is, even if lubrication between rolls (inter-roll lubrication) is employed in the 6-high mill disclosed in JP A 6-31304 in the manner as described in JP 5-50110, in a case where thrust force of the work rolls increases during rolling by any causes, it is difficult to decrease the thrust force by changing a cross angle of the intermediate roll.
Further, in the 6-high mill disclosed in JP A 6-31304, shape control of a plate is effected by changing a cross angle of the work rolls. However, the following was found, that is, changing of the cross angle of the work roll changes largely the thrust force occurring on the work roll, there occurs a difference between right and left loads with respect to an axial center of the work roll because of mill center moment changes, and the difference between the right and left loads becomes a cause of erroneous correction of thickness distribution of a rolling plate material.